1. Field of the Invention
This invention relates to the molding of plastic whitening or bleaching trays used in the practice of dentistry and orthodontics. More particularly, the invention relates to low-density polyethylene bleaching trays which are typically shaped from a low-density polyethylene plate or sheet of selected thickness by a thermoforming machine, to the dental specifications of a patient, for bleaching the patient's teeth to a desired degree of whiteness. The low-density thermoplastic polyethylene material is characterized by substantial transparency and comfort when worn, along with resiliency, excellent memory and thus superior "clinging" fit, as well as excellent wear characteristics. The low-density polyethylene material also cools rapidly after shaping on the thermoforming machine, does not shrink, and is therefore virtually immediately ready for application to the patient's teeth as a bleaching appliance.
Various types of plastics are well known for use, typically with pressure or vacuum thermoforming machines, to construct such dental and orthodontic-related devices as dental bleaching trays. Typical of these materials in the "0.020 Coping"(polypropylene), a hard, thin, opaque material that has a low melting point and good thermo-forming characteristics and EVA 0.020-0.080 inches, a soft, semi-clear material which is easily vacuum-formed and trimmed.
Generic fit, disposable, plastic impression, whitening or bleaching trays are well known in the art, which trays are built to selected generic size specifications, but not to the exact specifications of a user's teeth. These trays are typically constructed of material such as ethyl vinyl acetate (EVA). However, the generic trays suffer from the disadvantage of failing to closely conform to the teeth of the patient and therefore sometimes facilitate a less than satisfactory uniformity in brightening the teeth, since the whitening gel does not always contact all portions or margins of the tooth surface as intended. Moreover, the EVA in sheet form is cloudy and tends to distort after vacuum or pressure forming and tends to impede a wearer's speaking ability. The sheet material EVA is also easily bitten-through by the user. Ethyl vinyl acetate and polypropylene (coping) are also used to vacuum mold bleaching trays, with varying degrees of patient satisfaction.
It has surprisingly been found that low-density polyethylene is a superb material for use in making dental whitening or bleaching trays and typically in the vacuum or pressure thermoforming of the impression or bleaching trays to the exact specifications of a patient. While generally thought to be too soft and opaque or white for application to the construction of dental appliances, and bleaching trays in particular, the thermoplastic low-density polyethylene tray is surprisingly comfortable to wear, nearly transparent and invisible when thinly molded and has good chemical resistance to both diluted and concentrated acids, peroxides, carbapoles, alcohols, aldehydes, bases and esters. Furthermore, although flexible, the low density polyethylene tray is tough and not easily bitten through and maintains its "memory" and structural integrity throughout a temperature range of from about -50 degrees centigrade to about 100-degrees centigrade and allows clear speech when worn, due to its thinness. The pressure or vacuum-formed low-density polyethylene material also fits snugly against the cervical third of the tooth and the clearance between the gingiva and the top edge of the tray can be closely controlled in the desirable range from no more than about 1 mm. to about 1.5 mm. Since this temperature range is ideal for the construction and use of custom-designed bleaching tray appliances from low-density polyethylene, the thermoplastic polyolefin material has been found to be ideal for such application. Low-density polyethylene sheets or plates for use in vacuum or pressure thermoforming bleaching trays according to this invention are typically shaped from the polymenzation of polyethylene at relatively high pressure and the translucent pellets can be shaped into the sheets or plates of selected thickness, typically from about 0.020 to about 0.040 of an inch, for thermoforming into bleaching trays. The material easily and efficiently accommodates state-of-the-art whitening agents, including hydrogen peroxide, carbomide peroxide and the like.
Low-density polyethylene is the first of the polyolefins originally prepared by the high pressure (100-500 atmospheres) polymerization of ethylene and its comparatively low-density arises from the presence of a small amount of branching in the polymerized chain (only about 2% of the carbon atoms), which gives a more open chain structure. While the material is comfortable to wear and is normally translucent-to-opaque in color, it has further surprisingly been found that it becomes substantially transparent at thicknesses of about one-third of a millimeter, after being thermoformed on a dental impression cast to the specifications of the teeth of a patient. The material is, at the same time, tough and flexible and has excellent "memory" to provide a good fit over the patient's teeth after thermoforming, without distortion of the material. Accordingly, the molded low density polyethylene bleaching tray is well suited to accept and uniformly distribute bleaching gel over the margins and surfaces of the teeth after the bleaching gel is applied to the interior surfaces of the tray.
In contrast to the desirable characteristics described above with regard to low-density polyethylene, the materials polypropylene and ethylvinylacetate, two materials which are commonly used to mold dental bleaching trays, are less desirable. These materials present minimal "memory" and barely acceptable fit over time in positioning on and removing from a patient's teeth. While polypropylene is able to withstand higher temperatures than low-density polyethylene, its rigidity makes it much less suitable for repetitive fitting than low-density polyethylene for application to dental bleaching trays.
Accordingly, it is an object of this invention to provide a new and improved "user friendly" thermoplastic low-density polyethylene for molding into dental bleaching trays.
Another object of the invention is to provide a new and improved low-density polyethylene sheet or plate material of selected thickness for vacuum or pressure thermoforming on a dental impression and creating a plastic bleaching tray which conforms to the dental impression and accurately, comfortably and repetitively fits the teeth of a patient and may be self-applied.
Still another object of this invention is to provide a new and improved low-density polyethylene thermoplastic material which may be manufactured in sheets of sufficient thickness for molding in a pressure or vacuum thermoforming machine and production of custom-made bleaching trays.
A still further object of this invention is to provide a new and improved low-density polyethylene plastic which may be shaped into thin plastic plates or sheets for use in pressure or vacuum thermoforming machines, where the sheets are heat and vacuum-molded over custom-made dental impression casts for creating high quality dental bleaching trays that closely and comfortably match the contour and configuration of a patient's teeth and are substantially transparent as so molded.